By putting MONTH(Status...) on the Rows, Tableau build a vertical list with every values of those Month. Each month have a Mark that you set as a Pie Chart.
So when you say that Tableau stacks the charts vertically by default, it is not completely correct. It is not by default, it is by the order you set your dimension.
So just move MONTH(Status...) on the Columns, and you will have the list of MONTH horizontally, and the pies will follow.
Go to the sheet and press CTRL+W
you will get the horizontal layout of Pie charts
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